Silicon based substrate with environmental/thermal barrier layer

ABSTRACT

A barrier layer for a silicon containing substrate which inhibits the formation of gaseous species of silicon when exposed to a high temperature aqueous environment comprises a barium-strontium alumino silicate.

This is a Division of application Ser. No. 09/292,349, filed Apr. 15,1999 allowed.

The invention described herein was made in the performance of work underNASA Contract No. NAS3-26385, and is subject to the provisions ofSection 305 of the National Aeronautics and Space Act of 1958, asamended (42 U.S.C. 2457).

BACKGROUND OF THE INVENTION

The present invention relates to an article comprising a substratecontaining silicon and a barrier layer which functions as a protectiveenvironmental/thermal barrier coating and, more particularly, a barrierlayer which inhibits the formation of gaseous species of Si,particularly Si(OH)_(x) when the article is exposed to a hightemperature, aqueous (water and/or steam) environment.

Ceramic materials containing silicon and metal alloys containing siliconhave been proposed for structures used in high temperature applicationsas, for example, gas turbine engines, heat exchangers, internalcombustion engines, and the like. A particular useful application forthese materials is for use in gas turbine engines which operate at hightemperatures in aqueous environments. It has been found that thesesilicon containing substrates can recede and lose mass as a result of aformation volatile Si species, particularly Si(OH)_(x) and SiO whenexposed to high temperature, aqueous environments. For example, siliconcarbide when exposed to a lean fuel environment of approximately 1 ATMpressure of water vapor at 1200° C. will exhibit weight loss andrecession at a rate of approximately 6 mils per 1000 hrs. It is believedthat the process involves oxidation of the silicon carbide to formsilica on the surface of the silicon carbide followed by reaction of thesilica with steam to form volatile species of silicon such as Si(OH)_(x). Naturally it would be highly desirable to provide a externalbarrier coating for silicon containing substrates which would inhibitthe formation of volatile silicon species, Si(OH)_(x) and SiO, andthereby reduce recession and mass loss.

Accordingly, it is the principle object of the present invention toprovide an article comprising a silicon containing substrate with abarrier layer which inhibits the formation of gaseous species of Si,particularly Si(OH)_(x), when the article is exposed to a hightemperature, aqueous environment.

A second objective of this invention is to provide an article comprisinga substrate with a barrier layer providing thermal protection, suchlayer closely matching the thermal expansion of the substrate.

It is a further object of the present invention to provide a method forproducing an article as aforesaid.

SUMMARY OF THE INVENTION

The present invention relates to an article comprising a siliconcontaining substrate having a barrier layer on the substrate, whereinthe barrier layer functions to both inhibit the formation of undesirablegaseous species of silicon when the article is exposed to a hightemperature, aqueous environment and to provide thermal protection. Byhigh temperatures is meant the temperature at which the Si in thesubstrate forms Si(OH)_(x) and/or SiO in an aqueous environment. Byaqueous environment is meant a water and/or steam environment. Thesilicon containing composite is preferably a ceramic or metal alloycontaining silicon. The external barrier layer is characterized by acoefficient of thermal expansion which is within plus or minus 3.0 ppmper degree centigrade of the coefficient of expansion of the siliconcontaining substrate. The preferred barrier layer in accordance with thepresent invention is a barium aluminosilicate and, preferably, abarium-alkaline earth aluminosilicate wherein the alkaline earth metalis ideally strontium. In a preferred embodiment of the present inventionthe article can include one or more intermediate layers between thesilicon based substrate and the barrier layer. The intermediate layer(s)serve(s) to provide enhanced adherence between the barrier layer and thesubstrate and/or to prevent reactions between the barrier layer and thesubstrate.

The invention further relates to a method for producing an articlecomprising a silicon containing substrate and a barrier layer whichinhibits the formation of gaseous species of silicon and/or providesthermal protection when the article is exposed to a high temperature,aqueous environment as defined above.

Further objects and advantages of the present invention will appearhereinbelow from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the stability of the barrier layer of thepresent invention with respect to recession and mass loss;

FIG. 2 is a photomicrograph through a sample of the barrier layer of thepresent invention on a silicon carbide substrate;

FIG. 3 is a photomicrograph of a sample of the barrier layer of thepresent invention applied to an intermediate layer on a silicon carbidesubstrate; and

FIG. 4 demonstrates the effect of three specimens of the barrier layerof the present invention on weight loss in high temperature, aqueousenvironments.

DETAILED DESCRIPTION

The present invention relates to an article comprising a siliconcontaining substrate and a barrier layer, wherein the barrier layerinhibits the formation of gaseous species of silicon when the article isexposed to a high temperature, aqueous environment. The invention alsorelates to a method for producing the aforesaid article. In addition, itshould be appreciated that while the barrier is particularly directed toan environmental barrier layer, the barrier layer also functions as athermal barrier layer and thus the present invention broadly encompassesthe use of environmental/thermal barrier layers on silicon containingsubstrates and on substrates having comparable thermal expansioncoefficients.

According to the present invention, the silicon containing substrate maybe a silicon containing ceramic substrate or a silicon containing metalalloy. In a preferred embodiment, the silicon containing substrate is asilicon containing ceramic material as, for example, silicon carbide,silicon nitride, silicon carbon nitride, silicon oxynitride and siliconaluminum oxynitride. In accordance with a particular embodiment of thepresent invention, the silicon containing ceramic substrate comprises asilicon containing matrix with reinforcing such as fibers, particles,and the like and, more particularly, a silicon based matrix which isfiber-reinforced. Particularly suitable ceramic substrates are a siliconcarbide coated silicon carbide fiber-reinforced silicon carbide particleand silicon matrix, a carbon fiber-reinforced silicon carbide matrix anda silicon carbide fiber-reinforced silicon nitride matrix. Particularlyuseful silicon-metal alloys for use as substrates for the article of thepresent invention include molybdenum-silicon alloys, niobium-siliconalloys, and other Si containing alloys having a coefficient of thermalexpansion compatable with the barrier layer of the present invention.

Barrier layers particularly useful in the article of the presentinvention include alkaline earth metal aluminosilicates. In accordancewith a preferred embodiment, barium aluminosilicates, barium-alkalineearth metal aluminosilicates and, particularly, barium-strontiumaluminosilicates are preferred. In a particular embodiment, the barrierlayer comprises from about 0.00 to 1.00 mole BaO, from about 0.00 to1.00 mole of an oxide of a second alkaline earth metal, preferably SrO,about 1.00 mole Al₂O₃ and about 2.00 mole SiO₂, wherein the total of theBaO and the second alkaline earth metal or SrO is about 1.00 mole. Thepreferred barrier layer of the present invention comprises from about0.10 mole to about 0.9 mole, preferably 0.25 to about 0.75 mole BaO, 0.1mole to about 0.9 mole, preferably 0.25 to about 0.75 SrO, 1.00 moleAl₂O₃ and about 2.00 mole SiO₂ wherein the BaO and SrO total is about1.00 mole. A particularly suitable barrier layer for use on siliconcontaining ceramic compositions in the article of the present inventioncomprises about 0.75 mole BaO, about 0.25 mole SrO, about 1.00 moleAl₂O₃, and about 2.00 mole SiO₂.

It is an important feature of the present invention to maintaincompatibility between the coefficient of thermal expansion of thesilicon containing substrate and the barrier layer. In accordance withthe present invention it has been found that the coefficient of thermalexpansion of the barrier layer should be within ±3.0 ppm per degreescentigrade, preferably ±0.5 ppm per degrees centigrade, of thecoefficient of thermal expansion of the silicon containing substrate.When using a silicon containing ceramic substrate such as a siliconcarbide or a silicon nitride matrix with or without reinforcing fibersas described above in combination with the preferred barium-strontiumaluminosilicate barrier layer of the present invention, it is necessaryto develop a stable crystallographic structure in the barrier layer ofat least 50% by volume celsian in order to insure both structuralintegrity of the barrier layer and the desired thermal compatibilitywith respect to expansion coefficient between the silicon containingsubstrate and the barrier layer. The crystallographic structure of thebarium-strontium aluminosilicate barrier layer is obtained as a resultof preferred processing application and heat treating processing stepsas will be described hereinbelow.

The barrier layer should be present in the article at a thickness ofgreater than or equal to about 0.5 mils (0.0005 inch), preferablybetween about 3 to about 30 mils and ideally between about 3 to about 5mils. The barrier layer may be applied to the silicon based substrate byany suitable manner known in the art, however, it is preferable that thebarrier layer be applied by thermal spraying as will be describedhereinbelow.

In a further embodiment of the article of the present invention, anintermediate layer can be provided between the silicon containingsubstrate and the barrier layer. The intermediate layer(s) serve(s) toprovide enhanced adhesion between the barrier layer and the substrateand/or to prevent reactions between the barrier layer and the substrate.The intermediate layer consists of, for example, SiO₂, mullite,mullite-barium strontium aluminosilicate, mullite-yttrium silicate,mullite-calcium aluminosilicate, and silicon metal. Mullite has beenfound to be a particularly useful intermediate layer; however, mulliteby itself tends to be cracked as the result of thermal spray fabricationprocessing. Accordingly, it is preferred that the barrier layercomprises mullite-barium strontium aluminosilicate, mullite-yttriumsilicate, or mullite-calcium aluminosilicate in an amount of betweenabout 40 to 80 wt. % mullite and between about 20 to 60 wt. % bariumstrontium aluminosilicate or yttrium silicate or calciumaluminosilicate. The thickness of the intermediate layer is typical tothose described above with regard to the barrier layer and theintermediate layer may likewise be disposed in any manner known in theprior art, however, preferably by thermal spraying as describedhereinbelow.

In addition to the intermediate layer, a bond layer may be providedbetween the silicon containing substrate and the intermediate layer. Asuitable bond layer includes silicon metal in a thickness of 3 to 6mils. Alternatively, the silicon containing substrate may bepre-oxidized to provide a SiO₂ bond layer prior to application of theintermediate layer.

The method of the present invention comprises providing a siliconcontaining substrate and applying a barrier layer wherein the barrierlayer inhibits the formation of gaseous species of silicon when thearticle is exposed to a high temperature, aqueous environment. Inaccordance with the present invention it is preferred that the barrierlayer be applied by thermal spraying. It has been found that the barrierlayer should be thermal sprayed at a temperature of between about 870°C. to 1200° C. in order to help equilibrate the as-sprayed, splatquenched, microstructure and to provide a means to manage stresses whichcontrol delamination. When the article being prepared in accordance withthe method of the present invention is a silicon containing ceramic witha barium strontium aluminosilicate barrier layer, it is preferred thatthe barium strontium aluminosilicate barrier layer have celsiancrystallographic structure in an amount of at least 50% by volume in thebarrier layer. The formation of the celsian crystallographic structureinsures compatibility between the coefficient of thermal expansion ofthe silicon containing ceramic and the barium strontium aluminosilicatebarrier layer as described above.

The silicon containing substrate should be cleaned prior to applicationof the barrier layer to remove substrate fabrication contamination. Itis preferred that the silicon based substrate be subjected to a gritblasting step prior to application of the barrier layer. The gritblasting step must be carried out carefully in order to avoid damage tothe surface of the silicon-containing substrate such as silicon carbidefiber reinforced composite. It has been found that the particles usedfor the grit blasting should be hard enough to remove the undesiredcontamination but not as hard as the substrate material to preventerosive removal of the substrate. Further, the particles must be smallto prevent impact damage to the substrate. When processing an articlecomprising a silicon carbide ceramic composite substrate, it has beenfound that the grit blasting should be carried out with Al₂O₃ particles,preferably of a particle size of ≦30 microns and, preferably, at avelocity of about 150 to 200 m/sec. In addition to the foregoing, it maybe particularly useful to preoxidize the silicon based substrate priorto application of the intermediate and/or barrier layer in order toimprove adherence. It has been found that bond layers of between 100nanometers to 2000 nanometers are preferred. SiO₂ bond layers of thedesired thickness can be achieved by preoxidizing the silicon-carbidesubstrate at a temperature of between 800° C. to 1200° C. for about 15minutes to 100 hours.

The silicon bond layer may be applied directly to the grit blastedsurface by thermal spraying at approximately 870° C. to a thickness of 3to 6 mils.

Intermediate layers may be applied between the substrate and/or bondlayer and the barrier layer or between the bond layer and barrier layerby thermal spraying in the same manner described above with respect tothe barrier layer. As noted above, the preferred intermediate layersinclude mullite, mullite-barium strontium aluminosilicate,mullite-yttrium silicate, and mullite-calcium aluminosilicate.

After application of the desired layers to the silicon-based substratematerial, the article is subjected to a heat treatment step in order toprovide stress relief to the thermal sprayed structure, to promotebonding between the sprayed powder particles and between the layers andthe substrate, and to develop the celsian phase in the BSAS barrierlayer which is essentially amorphous in the as sprayed condition. Theheat treatment step is carried out at a temperature of about 1250° C.for about 24 hours.

The advantages of the article of the present invention will become clearfrom consideration of the following examples.

EXAMPLE 1

FIG. 1 shows a comparison of a hot pressed bulk specimen of BSAS, ofcomposition 0.75 BaO·0.25 SrO·Al₂O₃·2 SiO₂in comparison to siliconcarbide thermal cycle tested 250 cycles between room temperature and1200° C. in a simulated combustion environment of high steam, lean fuelconditions. The results show 8 mg/cm² weight loss for the siliconcarbide while the BSAS gains a very small amount of weight ˜0.4 mg/cm².The results show that silicon carbide is not stable to this environmentand that the BSAS system is much more stable.

EXAMPLE 2

FIG. 2 is a cross section of a 4 mil thick BSAS of composition 0.75BaO·0.25 SrO·Al₂O₃·2 SiO₂ coating on SiC composite. The BSAS was thermalsprayed onto the silicon carbide composite using the followingparameters:

Parameter Setting Plasma torch Metco 3M Nozzle GH Anode std. Powder portMetco #2 Primary gas Ar@80 Metco gage Secondary gas H2@8 Metco gageSubstrate temp. 850° C. Carrier gas Ar@37 Metco gage Powder feed 15 to25 gpm Intermed. Surface Power 3Okw 25kw Stand-off 2.5—3′′ 5′′

Prior to coating the substrate was cleaned by grit blasting with 27micron alumina particles at an impact velocity of 150 to 200 mps. As canbe seen from FIG. 2, the invention results in an excellent barrier layerstructure.

EXAMPLE 3

FIG. 3 is a cross sectional view of a BSAS of composition 0.75 BaO·0.25SrO·Al₂O₃ ·2 SiO₂ barrier layer on a mullite/BSAS intermediate layer of4±1 mils thickness on a silicon layer on silicon carbide composite. Thecoating was fabricated using the following parameters.

Plasma torch Metco 3M Nozzle GH Anode std. Powder port Metco #2 Primarygas Ar@80 Metco gage Secondary gas H2@8 Metco gage Substrate temp. 850°C. Carrier gas Ar@37 Metco gage Powder feed 15 to 25 gpm InterfaceIntermed. Surface Power 25 kw 30kw 25kw Stand-off 4′′ 2.5—3′′ 5

Prior to coating the substrate was cleaned by grit blasting with 27micron alumina particles at an impact velocity of 150 to 200 mps. As canbe seen from FIG. 3, the invention results in an excellent barrier layerstructure.

EXAMPLE 4

Coatings of the BSAS (barium strontium aluminosilicate) based barrierlayer coating system were fabricated onto silicon carbide compositesubstrates having an intermediate layer as indicated below and wereexposed along with an uncoated silicon carbide composite substrate(98-17A) to high pressure, combustion environment, burner rig testingsimilar to conditions that occur in gas turbines engines. The BSAScoatings on all samples had the following composition: 0.75 BaO·0.25SrO·Al₂O₃ ·2 SiO₂. The BSAS coatings were 4±1 mils in thickness and theintermediate layers were also 4±1 mils in thickness and were applied bythermal spraying as indicated below. These coatings consisted ofvariations of BSAS on mullite (98-17C) and BSAS on mullite plus BSAS(98-17B and 98-17D). Testing occurred using conditions of 1200° C. testtemperature, 200 hours exposure time, a fuel to air ratio of 0.053, and6 atm pressure. After 200 hours exposure the uncoated substrateexhibited approximately 65 mg weight loss compared with weight gain ofthe coated coupons demonstrating that the coatings protected thesubstrate based on weight change data. No bond coat was used for thesespecimens. Thermal spray parameters were:

Parameter Setting Plasma torch Electro-plasma 03CA Nozzle 03CA-27 Anode03CA-167 Powder port ext. 9O° Primary gas Ar@14.4 SLM Secondary gasHe@9.8 SLM Substrate temp. 1O5O—125O° C. Carrier gas Ar@3—6 SLM Powderfeed 20 gpm Intermed. Surface Power 45kw 45kW Stand-off 4′′ 4′′

The results are shown in FIG. 4 below which shows the effectiveness ofthe barrier layer of the present invention.

The coefficient of thermal expansion (CTE) was measured for BSAS systemof 0.75 BaO·0.25 SrO·Al₂O₃·2 SiO₂ having different celsian contents.Celsian content was determined by x-ray analysis of hot pressed couponsof BSAS. The celsian content affected the CTE as seen below.

% Celsian Content CTE 5 8.1 25 7.4 95 5.2

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:
 1. A method for preparing an article comprising thesteps of: providing a substrate comprising silicon; and applying agaseous species of Si inhibiting barrier layer to the substrate whereinthe barrier layer has a crystallographic structure which is at least5.0% by volume celsian and inhibits the formation of a gaseous speciesof Si when the article is exposed to a high temperature, aqueousenvironment and wherein the coefficient of thermal expansion of thebarrier layer is within ±3.0 ppm/° C. the coefficient of thermalexpansion of the substrate.
 2. A method according to claim 1 wherein thecoefficient of thermal expansion of the barrier layer is within ±0.5ppm/° C. the coefficient of thermal expansion of the substrate.
 3. Amethod according to claim 1 further including the step of grit blastingthe substrate prior to applying the barrier layer.
 4. A method accordingto claim 3 including grit blast with alumina particles having a particlesize of ≦30 microns.
 5. A method according to claim 4 including gritblasting at a velocity of about 150/m/sec to about 200/m/sec.
 6. Amethod according to claim 1 including applying the barrier layer bythermal spraying.
 7. A method according to claim 6 including thermalspraying while holding the substrate at a temperature of between about870° C. to 1200C.
 8. A method according to claim 1 including the step ofpreoxidizing the substrate to form a layer of SiO₂ prior to applying thebarrier layer.
 9. A method according to claim 8 wherein the preoxidizingcomprises heating the substrate at a temperature of between about 800°C. to 1200° C. for about 15 minutes to 100 hours.
 10. A method accordingto claim 8 wherein the coefficient of thermal expansion of the barrierlayer is within ±0.5 ppm/° C. the coefficient of thermal expansion ofthe substrate.
 11. A method according to claim 1 including the step of,after applying the barrier layer, heat treating the article at atemperature of about 1250° C. for about 24 hours.
 12. A method accordingto claim 1 including heat treating at a temperature of about 12500° C.for about 24 hours.
 13. A method for preparing an article comprising thesteps of: providing a substrate comprising silicon; grit blasting thesubstrate with particles having a particle size ≦30 microns and applyinga gaseous species of Si inhibiting barrier layer to the substratewherein the barrier layer inhibits the formation of a gaseous species ofSi when the article is exposed to a high temperature, aqueousenvironment.
 14. A method for preparing an article according to claim 13wherein the particles are alumina particles.
 15. A method according toclaim 13 or claim 14 including grit blasting at a velocity of about100/m/sec to about 200/m/sec.
 16. A method according to claim 1 or 13wherein the barrier layer comprises from about 0.00 to 1.00 moles BaO,from about 0.00 to 1.00 mole SrO, about 1.0 mole Al₂O₃ and about 2.00mole SiO₂, wherein the total of BaO and SrO is about 1.00 mole.
 17. Amethod according to claim 1 or wherein the barrier layer consistsessentially of from about 0.00 to 1.00 mole BaO, from about 0.00 to 1.00mole of an oxide of a second alkaline earth metal, about 1.00 mole Al₂O₃and about 2.00 mole SiO₂, wherein BaO plus the other alkaline earthmetal oxide total 1 mole.
 18. A method according to claim 1 or 13wherein the barrier layer comprises from about 0.10 mole to about 0.90mole BaO and from about 0.10 mole to about 0.90 mold SrO.
 19. A methodaccording to claim 1 or 13 wherein the barrier layer comprises fromabout 0.25 mole to about 0.75 mole BaO and from about 0.25 mole to about0.75 mole SrO.
 20. A method according to claim 1 or 13 wherein thebarrier layer comprises about 0.75 mole BaO and about 0.25 mole SrO. 21.A method for preparing an article comprising the steps of: providing asubstrate comprising silicon; oxidizing the substrate to form a layer ofSiO₂; applying a gaseous species of Si inhibiting barrier layer to theoxidized substrate wherein the barrier layer inhibits the formation ofgaseous species of Si when the article is exposed to a high temperature,aqueous environment.
 22. A method according to claim 21 wherein thepreoxidizing comprises heating the substrate at a temperature of betweenabout 800° C. to 1200° C. for about 15 minutes to 100 hours.
 23. Amethod according to claim 21 wherein the coefficient of thermalexpansion of the barrier layer is within ±3.0 ppm/° C. the coefficientof thermal expansion of the substrate.
 24. A method according to claim21 wherein the coefficient of thermal expansion of the barrier layer iswithin ±0.5 ppm/° C. the coefficient of thermal expansion of thesubstrate.
 25. A method according to claim 21 wherein the barrier layerhas a crystallographic structure which is at least 50% by volumecelsian.
 26. A method for preparing an article comprising the steps of:providing a substrate comprising silicon; applying a gaseous species ofSi inhibiting barrier layer to the substrate wherein the barrier layerinhibits the formation of gaseous species of Si when the article isexposed to a high temperature, aqueous environment; and heat treatingthe article at a temperature of about 1250° C. for about 24 hours.
 27. Amethod for preparing an article comprising the steps of: providing asubstrate comprising silicon; and applying a gaseous species of Siinhibiting barrier layer to the substrate wherein the barrier layer hasa crystallographic structure which is at least 5.0% by volume celsianand consists essentially of from about 0.00 to 1.00 mole BaO, from about0.00 to 1.00 mole of an oxide of a second alkaline earth metal, about1.00 mole Al₂O₃ and about 2.00 mole SiO₂, wherein BaO plus the otheralkaline earth metal oxide total 1 mole and inhibits the formation ofgaseous species of Si when the article is exposed to a high temperature,aqueous environment.
 28. A method for preparing an article comprisingthe steps of: providing a substrate comprising silicon; and applying agaseous species of Si inhibiting barrier layer to the substrate whereinthe barrier layer is selected from the group consisting of (a) bariumaluminosilicate, (b) barium strontium aluminosilicate, (c) from about0.00 to 1.00 moles BaO, from about 0.00 to 1.00 mole SrO, about 1.0 moleAl₂O₃ and about 2.00 mole SiO₂ wherein the total of BaO and SrO is about1.00 mole and (d) from about 0.00 to 1.00 mole BaO, from about 0.00 to1.00 mole of an oxide of a second alkaline earth metal, about 1.00 moleAl₂O₃ and about 2.00 mole SiO₂, wherein BaO plus the other alkalineearth metal oxide total 1 mole wherein the barrier layer has acrystallographic structure which is at least 50% by volume celsian andinhibits the formation of a gaseous species of Si when the article isexposed to a high temperature, aqueous environment.